The present invention relates to a method for manufacturing heat transfer element sheets from a roll of sheet material for stacking within an element basket of the type typically utilized in a rotary regenerative heat exchanger. More particularly, the present invention relates to a method for manufacturing the heat transfer sheets to facilitate stacking within the element basket wherein the cutting of the sheet material is controlled to produce sheets which when stacked alternately in juxtaposition within the element basket will not nest.
A typical rotary regenerative heater has a cylindrical rotor divided into compartments in which are disposed sheets of heat transfer element supported in various element baskets mounted in the rotor. The element baskets comprise open-ended housings such as, but not limited to, the element baskets disclosed in U.S. Pat. Nos. 2,432,198; 3,314,472; and 3,379,340. As the rotor turns, the heat transfer element sheets disposed in the element baskets are alternately exposed to a stream of heating gas and then upon rotation of the rotor to a stream of cooler air or other gaseous fluid to be heated. As the heat transfer element sheets are exposed to the heating gas, they absorb heat therefrom and then when exposed to the cool air or other gaseous fluid to be heated, the heat absorbed from the heating gas by the heat transfer element sheets is transferred to the cooler gas.
Typically, the heat transfer element sheets are closely stacked in spaced relationship within the element basket to provide a plurality of passageways between juxtaposed plates for the flow of the heat exchange fluid therebetween. Typically, the heat transfer element sheets are crimped at frequent and generally equally spaced intervals to provide notches which extend outwardly from the sheet for spacing the stacked sheets from each other. The spacers formed in the heat transfer element sheet may be single-lobed notches extending outwardly from the sheet in one direction or double-load notches extending outwardly from the sheet in opposite directions as shown in U.S. Pat. No. 2,596,642, or the spacers may comprise a plurality of dimples formed in the sheet as disclosed in U.S. Pat. No. 4,363,222. When the sheets are stacked together in juxtaposition within the element basket, these notches or spacers serve not only to maintain adjacent sheets at their proper spacing, but also to provide support between adjacent sheets so that the force that is placed on the sheets by the flow of heating fluid through the baskets, and also the flow of pressurized cleaning gas through the baskets, can be equilibrated between the various sheets making up the heat transfer element assembly housed within the basket.
In forming the heat transfer element basket assemblies, the heat transfer element sheet material is drawn from a coil by a continuous rolling process and passed through at least one press which crimps the notches or spaces into heat transfer element sheet. The individual sheets are then cut sequentially from the notched sheet. As the element baskets are typically in the form of an open-ended right trapazoidal prism, each of the individual sheets will be of a different length so that when the inidividual sheets are stacked endwise within the element basket, they will fit snugly therein. As the individual sheets are cut from the roll of sheet material, they are placed endwise in the element basket in juxtaposition with each other.
Because of the varying length of the individual sheets, it typically occurs in the typical prior art cutting process that one or more of the sheets making up the heat transfer element basket assembly will be cut at a length such that its notches coincide with the notches of an adjacent sheet. This results in the notches in the adjacent sheets nesting thereby destroying the spacing between the two sheets. When this occurs, the operator must stop the cutting process and hand operate the machine to cut a new plate to substitute for one of the nesting plates in such a manner that the notches will not line up. After this has been accomplished, the operator may reinitiate the automatic cutting process. However, the shutting down of the automatic process and the hand cutting of a replacement sheet can be a timely and therefor costly endeavor.
A recently developed process for automatically cutting heat transfer element sheets in such a manner as to preclude the possibility of nesting between adjcent sheets when the sheets are stacked in the element basket is disclosed in assignee's co-pending application, Ser. No. 487,553, filed Apr. 22, 1983. As disclosed therein, the cutting process is controlled such that a plurality of first sheets are cut from the notched sheet of material so that each of the first subsheets has a leading edge sheared along a first line spaced a fixed distance from the nearest notch therein and so that each of the second subsheets has a leading edge sheared along a second line spaced a fixed distance from the nearest notch therein, the fixed distances being different by a fixed amount equal to a fraction of the interval between the equally spaced notches, preferably equal to one-half of that interval. Although such a cutting process does insure that juxtaposed subsheets will not nest when stacked in an element basket, a substantial amount of material is wasted when the first and second sheets are cut from a continuous sheet of material.